docs: def columns_used(self):
          """
          Returns all the columns used across all models in the group
          for filtering and in the model expression.
  
          """
          return list(tz.unique(tz.concatv(
              util.columns_in_filters(self.fit_filters),
              util.columns_in_filters(self.predict_filters),
              util.columns_in_formula(self.default_model_expr),
              self._group.columns_used(),
              [self.segmentation_col])))